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Hi Chris.
It is certainly not the standard Strathearn Sea urchin, which has 20 ribs with the sand grains very near the outer surface. I recall having a similar weight to yours, but without a label, and after some head scratching decided mine was probably Strathearn but of no known name. In the 'experimental abstracts' group on Richard's site there is a paperweight made in a similar 8 lobe mould, but filling more of the weight.
Alan